1. Field of the Invention
This invention relates to a method of manufacturing a water-permeable hydrophobic substrate and, more particularly, to the use of a transitory hydrophilic surfactant in an aqueous dispersion of a hydrophobic binder, depositing the binder on the substrate and negating the hydrophilicity of the surfactant.
2. Brief Description of the Prior Art
Water based binder systems are preferred because of the low cost associated with using water as a solvent, as compared to using, for example, an organic solvent such as ethanol. Water presents no recovery or pollution problem and, thus further contributes to its value as a solvent.
However, there is a conflict in that where an end product having a hydrophobic quality is produced through the use of a hydrophobic binder such as styrene-butadiene copolymers, since the styrene-butadiene latex must be free of surfactants or at least sufficiently surfactant-free so as not to interfere with the hydrophobicity of the end product.
By way of example, in the case of a disposable diaper, an absorbent wadding would be produced as disclosed in U.S. Pat. No. 3,180,335, the disclosure of which is herein incorporated by reference. The web is a compliant, soft-feeling, porous, hydrophobic paper or non-woven fabric member. An example of a non-woven fabric sheet which has been found to function well on diapers is one which comprises 1.5 to 3 denier rayon and contains approximately 20-35% thermoplastic binder (as, for example, copolymers of an ester of acrylic acid such as is sold by the Rohm & Haas Company and identified as B-15), and having a weight of about 15-19 grams per square yard. The patent discloses that, for best results in processing, such a sheet, surfactants should be minimal in the binder emulsion and avoided in the final bath. The patent further discloses the use of a flexible waterproof back sheet desirably comprising a 0.001" thick low density, opaque polyethylene web which is preferably larger than the corresponding dimensions of the pad.